The present invention relates generally to radiation sensitive photoresist compositions and particularly to compositions containing aqueous alkali soluble resins together with naphthoquinone diazide sensitizing agents.
It is well known in the art to produce positive photoresist formulations such as those described in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470. These include alkali-soluble phenol-formaldehyde novolak resins together with light-sensitive materials, usually a substituted diazonaphthoquinone compound. The resins and sensitizers are dissolved in an organic solvent or mixture of solvents and are applied as a thin film or coating to a substrate suitable for the particular application desired.
The resin component of these photoresist formulations is soluble in aqueous alkaline solutions, but the naphthoquinone sensitizer acts as a dissolution rate inhibitor with respect to the resin. Upon exposure of selected areas of the coated substrate to actinic radiation, however, the sensitizer undergoes a radiation induced structural transformation and the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in an alkaline developing solution while the unexposed areas are largely unaffected, thus producing a positive relief pattern on the substrate.
In most instances, the exposed and developed photoresist on the substrate will be subjected to treatment by a substrate-etchant solution. The photoresist coating protects the coated areas of the substrate from the etchant and thus the etchant is only able to etch the uncoated areas of the substrate, which in the case of a positive photoresist, correspond to the areas that were exposed to actinic radiation Thus, an etched pattern can be created on the substrate which corresponds to the pattern on the mask, stencil, template, etc., that was used to create selective exposure patterns on the coated substrate prior to development.
The relief pattern of the photoresist on the substrate produced by the method described above is useful for various applications including as an exposure mask or a pattern such as is employed in the manufacture of miniaturized integrated electronic components.
The properties of a photoresist composition which are important in commercial practice include the photospeed of the resist, development contrast, resist resolution, and resist adhesion.
Resist resolution refers to the capacity of a resist system to reproduce the smallest features from the mask to the resist image on the substrate.
In many industrial applications, particularly in the manufacture of miniaturized electronic components, a photoresist is required to provide a high degree of resolution for very small features (on the order of two micron or less).
The ability of a resist to reproduce very small dimensions, on the order of a micron or less, is extremely important in the production of large scale integrated circuits on silicon chips and similar components. Circuit density on such a chip can only be increased, assuming photolithography techniques are utilized, by increasing the resolution capabilities of the resist.
Photoresists are generally categorized as being either positive working or negative working. In a negative working resist composition, the imagewise light struck areas harden and form the image areas of the resist after removal of the unexposed areas with a developer. In a positive working resist the exposed areas are the non-image areas. The light struck parts are rendered soluble in aqueous alkali developers. While negative resists are the most widely used for industrial production of printed circuit boards, positive resists are capable of much finer resolution and smaller imaging geometries. Hence positive resists are the choice for the manufacture of densely packed integrated circuits.
In the normal manner of using a positive photoresist, a single layer of this material is imaged to give a mask on the substrate, which can further be etched with a suitable etchant or used for deposition of materials, such as metals. However, due to the limitations of optical imaging systems, resolution of small patterns, on the order of 2um or less, is limited, particularly if topography is present on the substrate. It was discovered by B.J. Lin and T.H.P. Chang, J. Vac. Sci. Tech. 1979, 16, p1669, that this resolution can be further improved by using multilevel systems to form a portable conformable mask.
In multilevel lithography a relatively thick layer of photoresist is used such that the substrate is covered with a flat layer of the photoresist. A different type of photoresist, in particular, one absorbing at a different wavelength as compared to the bottom layer, is then placed on top. This top layer is imaged through a mask using the actinic wavelength for the top layer resist. The image is developed using an appropriate developer. The top layer image can now be used as a portable conformable mask for imaging the bottom layer by flood exposing the system at the actinic wavelength of bottom photoresist. This system represents a simple bilevel scheme.
However, if undesirable intermixing or scumming is observed between the two photoresist layers then another barrier layer may be added between these two layers. The barrier layer may be an organic polymer that does not mix with either of the two layers, such as polyvinyl alcohol, or be an inorganic layer, such as silicon oxide or silicon nitride. From a processability point of view a bilevel system is more attractive than a trilevel one although a trilevel system is also contemplated by this invention.
The present invention relates to a multilevel system, where a layer of a first positive photoresist is coated onto a substrate. It is necessary for this invention that the bottom layer of photoresist be one capable of a controllable degree of crosslinking. The compositions described in U.S. patent applications Ser. No. 764,700, filed Aug. 12, 1985; Ser. No. 889,032, filed July 23, 1986; and Ser. No. 895,609, filed Aug. 11, 1986 are preferred and these applications are incorporated herein by reference. These compositions undergo tone inversion from positive to negative if the exposure energy is low, on the order of about 5-50 mJ/cm.sup.2, and an appropriate post-exposure bake is then used. The dosage and the temperature of the post-exposure bake determine the extent of crosslinking taking place in the photoresist and thus the dissolution properties of the image reversed photoresist. If no post-exposure bake is used, the photoresist behaves as a conventional positive resist. Crosslinking can also be introduced with only a bake and without exposure if a suitable crosslinking additive is used such that the system requires no photogenerated acidic species to induce crosslinking. For the purposes of this invention the conditions used for crosslinking are such that substantially no intermixing is observed when the top layer of photoresist is in place. This lower level resist is soft-baked at a temperature to largely remove the solvent from the film. The film is then flood exposed with u.v. light and baked such that the material is sufficiently crosslinked to prevent intermixing when the top layer of photoresist is deposited. The actinic wavelength for this photoresist may be, for example, 300-410nm.
If the substrate is highly reflective or contains topography, a dye may be added to the resist which forms the bottom layer. The absorption of the dye should be at the same wavelength as that used to image the top layer. The concentration of the dye should be sufficient to eliminate reflections from the substrate when imaging the top layer.
On top of the crosslinked lower resist is deposited a second photoresist that is actinic at a wavelength which is not absorbed by the sensitizer of the bottom resist. This may be any wavelength excluding, for example, 300nm to 410nm. In one case this could be 436nm. To the upper resist may be added a dye that is used to form a mask for the bottom layer, i.e. a dye that absorbs between 300nm to 410nm. The concentration of this dye should preferably be sufficient such that for the desired film thickness (300 .mu.m to 410 nm) the transmission at the exposure wavelength of the bottom resist is preferably less than 1%. This is to enable the top layer to behave as an efficient mask.
Imaging of the top layer may be done with, for example, 436nm filtered mercury light through a mask. The image is then developed with an aqueous alkaline developer, e.g., AZ 400K available commercially from American Hoechst Corporation. The complete system is then flood exposed with light at the actinic wavelength of the bottom resist. The wavelength of the light should also be such that the light is sufficiently absorbed by the dye in the patterned top layer to reduce the transmission in the masked areas to less than about one percent. The exposure dose has to be sufficiently high to convert enough of the naphthoquinone sensitizer to an indene carboxylic acid to overcome the insolubility of the previously crosslinked bottom resist. Exposure doses of about 100mJ/cm.sup.2 to about 300mJ/cm.sup.2 are sufficient. The bottom layer is then developed with an aqueous alkaline developer. Thus the image of the mask is transferred "positively" to the substrate if the top resist is a positive acting one.
This system has an advantage over other current bilevel systems in that no organic solvents need be used in the development stage. Aqueous alkaline developers may be used to develop both layers of this multilayer system. The bottom layer does not dissolve at an appreciable rate, since the bottom layer is partially crosslinked. In addition, since it is a bilevel system, it is much simpler than a trilevel one and requires less processing. Also without any further processing step the bottom layer possesses greater thermal stability than a conventionally positively imaged photoresist would, since it is partially crosslinked. This enhanced thermal stability is important for any etching where temperatures in the area of 150.degree. C. can be reached.
The invention uses a unique partially cross-linkable chemical composition, which when processed in a slightly modified manner to the usual and customary method of lithographic processing, yields a totally unexpected positive working bi-level resist system.
Among the advantages realized by this highly desirable result are improvement in the relationship between exposure energy and resulting line width, improvement in developed image resolution, substantial elimination of reflective notching and a totally aqueous developable system.